Well, you can click there names when they comment or post on the forums, and add them like that, or you could do this:
http://armorgames.com/user/name
Do you see where it says 'name'? Instead of that, you would add the name of the user that you would like to look up. You do that, and it will lead you to their profile page. From there, you could press 'add friend' and they will be added.